Made modifications to model

1. Added super class PASVGElement
2. PAShape, PASVGGroup(will merge with PAGroup soon) extends
PASVGElement
3. Added interface PAFillable which is implemented in PARect and
PACircle only
4. Removed PAFillableShape abstract class
5. Commented methods in PASVGContainer as previous ways have to change
to suit current ways
6. Changed LinkedList<PAShape> to LinkedList<PASVGElement> in
PASVGContainer, PAFileChooserAction and PASVGContainer